Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for acquiring seismic data having ghost and polarity diversity which is suitable for deghosting high- and medium-frequency data.
Discussion of the Background
Seismic reflection is a method of geophysical exploration to determine the properties of a portion of the earth's subsurface, information that is especially helpful in the oil and gas industry. Marine seismic reflection is based on the use of a controlled source that sends energy waves into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to estimate the depth and/or composition of the features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
For marine applications, a seismic survey system 100, as illustrated in FIG. 1, includes a vessel 102 that tows plural streamers 110 (only one is visible in the figure) and a seismic source 130. Streamer 110 is attached through a lead-in cable (or other cables) 112 to vessel 102, while source array 130 is attached through an umbilical 132 to the vessel. A head float 114, which floats at the water surface 104, is connected through a cable 116 to a streamer head junction 110A to streamer 110, while a tail buoy 118 is connected, through a similar cable 116, to a tail end 110B of streamer 110. Head float 114 and tail buoy 118 are used, among other things, to maintain the streamer's depth and to provide surface reference for positioning the underwater equipment. Seismic receivers 122 are distributed along the streamer and are configured to record seismic data. Seismic receivers 122 may include a hydrophone, geophone, accelerometer, gradient pressure receiver or a combination thereof. Positioning devices (birds and acoustics) 128 are attached along the streamer and interact with their respective controllers 126 and ranging devices 129 for adjusting a position of the streamer according to a survey plan.
Source array 130 has plural source elements 136, which are typically air guns. Alternatively, the source elements may be vibrators. The source elements are attached to a float 137 to travel at desired depths below the water surface 104. During operation, vessel 102 follows a predetermined path (or navigation track) T while source elements 136 emit acoustic waves 140. These waves bounce off the ocean bottom 142 and other layer interfaces below the ocean bottom 142 and propagate as reflected/refracted waves 144 that are recorded by receivers 122. The positions of both the source element 136 and recording receiver 122 are estimated based on GPS systems 124, acoustic devices 129 and depth controllers (birds) 126 and are recorded together with the seismic data in a storage device 127 onboard the vessel. Controller 126 is typically connected to the vessel's navigation system and other elements of the seismic survey system, e.g., birds 128.
Receiver 122 traditionally records sound pressure generated by wave 144 (primary). However, down-going waves 150 (ghosts) are also recorded by receiver 122, which interferes with up-going wave and mask the true signal 144. Ghost 150 originates from seismic source 130, but suffers a negative water reflection at water surface 104 (which results in a phase shift that modifies its polarity), which flips the polarity of seismic wave besides the subsurface reflection on layer 142 or other subsurface layer. Thus, ghost 150 propagates from the water surface toward receiver 122, while primary 144 propagates from the subsurface toward receiver 122. In this way, ghost 150 is recorded together with the primary by the same receiver, producing notches in the seismic data, which is undesirable.
To unmask the true signal 144 (i.e., deghost the data), various methods have been proposed in the art. One such deghosting process has been disclosed, for example, in U.S. Pat. No. 8,456,951 (herein '951) authored by R Soubaras, the entire content of which is incorporated herein. According to the '951 patent, a method for deghosting uses joint deconvolution for migration and mirror migration images to generate a final image of a subsurface. Deghosting is performed at the end of processing (during an imaging phase) and not at the beginning, as with traditional methods. Further, the '951 patent discloses that no datuming step (i.e., reconstructing the data at another depth) is performed on the data. Other deghosting methods have been proposed in the art.
However, there is a need for a new deghosting method that further removes the ghost from recorded seismic data for obtaining better images.